Alumina ceramic, either translucent or clear, is particularly suitable as an envelope material for high intensity alkali metal vapor lamps such as high pressure sodium vapor lamps, because it withstands the attack of the vapors of the alkali metals even at high temperatures. In making such lamps, end closures which support the electrodes must be bonded and hermetically sealed to a ceramic tube, and for this purpose a sealing glass or composition is commonly used. The closures may take the form of metal end caps which provide direct electrical connections to the electrodes. More recently ceramic plugs have been used for the closures because they are cheaper, and in such case a metal conductor must be sealed through the plug to provide the electrical connection. The metal having a coefficient of expansion closest to that of alumina ceramic is niobium, and it is the one generally used either for the end cap, or for the inlead in the case of a ceramic plug
Compositions commonly used for sealing high pressure sodium vapor lamps lie in the Al.sub.2 O.sub.3 -CaO-BaO system, typical ranges being 35-60 wt% Al.sub.2 O.sub.3, 30-60 wt% CaO, and 5-20 wt% BaO. Desirable properties of a sealing compositions are a thermal expansion closely matching that of the alumina ceramic having an average thermal expansion coefficient .varies. of 86.times.10.sup.-7 /.degree.C., a favorable liquidus temperature, that is one above the highest temperature to be encountered during operation of the lamp, wide sealing range, ability to form glasses when melted and cooled rapidly, and stability in the presence of alkali metal vapors at elevated temperatures. A composition meeting the foregoing requirements is disclosed in U.S. Pat. No. 3,588,577--McVey et al. It is centered on 45 wt% Al.sub.2 O.sub.3, 36 wt% CaO, 14 wt% BaO, and 5 wt% MgO, and, in the glassy form, has an average thermal expansion coefficient .varies. of 88.times.10.sup.-7 /.degree.C. between 25.degree. C. and 600.degree. C. This composition, referred to as G45, has been widely used by applicants' assignee in the commercial manufacture of high pressure sodium vapor lamps of the kind using niobium metal end caps. An improved composition disclosed in application Ser. No. 851,439 of McVey and Datta, filed Nov. 14, 1977 and similarly assigned is centered on 47 wt% Al.sub.2 O.sub.3, 37 wt% CaO and 16 wt% BaO, and has an average thermal expansion coefficient .varies. of 89.times.10.sup.-7 /.degree.C. This composition, referred to as G47, has more recently been used by applicants' assignee in the commercial manufacture of high pressure sodium vapor lamps of the kind using ceramic end plugs. However even when using the improved G47 sealing composition, some defective products in the form of cracked or leaky seals are produced in manufacture. Such defective products must be rejected and the proportion thereof is commonly referred to as the shrinkage rate.
The object of the invention is to provide a new composition for sealing to alumina ceramic, that is for sealing either metal or ceramic to alumina ceramic, which is superior to known compositions. In particular, a composition is desired providing a lower shrinkage rate when sealing ceramic plugs into the ends of alumina ceramic tubes in electric lamp manufacture.